Many conventional FeRAM devices include a horizontal ferrocapacitor structure, in which a stack of layers is formed including top and bottom electrodes sandwiching a ferroelectric layer. An alternative “vertical capacitor” structure was suggested in U.S. Pat. No. 6,300,652, the disclosure of which is incorporated herein by reference. A vertical capacitor includes a ferroelectric element sandwiched between electrodes to either side, all at substantially the same level in the FeRAM device.
The process steps of a conventional technique for forming a vertical capacitor structure are illustrated in FIGS. 1 to 5. The vertical capacitors are typically formed over a substructure, which may for example be of the form shown in FIG. 1 in which various electronic components 1 are connected to conductive plugs 3 which extend upwards through a matrix 5 (e.g. of TEOS (tetraethylorthosilicate)). The upper ends of the plugs 3 terminate in TiN/Ir barrier elements 7, having a top surface flush with the surface of the matrix 5.
As shown in FIG. 2, an insulating layer 9 of Al2O3 is formed over the surface of the matrix 5, and a layer of ferroelectric material 11 such as PZT (PbZrTiO3) is formed over that.
As shown in FIG. 3, hardmask elements 13 are deposited in selected areas of the PZT layer 11, and the portions of the PZT and Al2O3 which are not protected by the hardmask elements 13 are etched all the way through, forming openings 17. During this process Al2O3 fences 15 are often formed on the sides of the remaining PZT.
The openings 17 are then filled with conductive material 19 such as IrO2, by depositing IrO2 over the entire structure, as shown in FIG. 4, and chemical-mechanical planarization (CMP) polishing is performed to form a flat upper surface 21 which is partly the PZT 11 and partly the conductive material 19. Then, as shown in FIG. 5, an Al2O3 layer 23 is formed over the surface 21. The elements 19 of IrO2 constitute electrodes, while the remaining PZT 11 forms the dielectric.
The vertical capacitor structure has great potential for reducing the cell size, especially if the etching taper angle of the remaining PZT 11 (i.e. the angle between the horizontal direction and the sides of the remaining PZT) is high. However, if the taper angle becomes close to 90°, the Al2O3 fences 15 are more likely to be formed. These fences 15 are difficult to remove (e.g. by a wet cleaning process), and dramatically reduce the QSW (i.e. the maximum charge which can be stored in the ferrocapacitor) because the insulating fences 15 reduce the effective area of the capacitor.